Several distinct arterial blood-pressure parameters yield medically useful information, among them pressure at systole, pressure at diastole, mean arterial pressure, pulse pressure, and continuous arterial pressure. The traditional ways of measuring these may be categorized as follows: sphygmomanometry (cuff measurement), automated sphygmomanometry, and indwelling arterial-line transduction (A-line).
The importance of continuous arterial blood pressure as a medical indicator has spurred the development of new methods of measuring it. These include external pressure transduction, photoplethysmography, and pulse-wave transit timing. To date these latter methods have been used mainly experimentally.
Sphygmomanometry, the most widely used traditional method, gives pressure at systole and pressure at diastole. The automated cuff uses a machine-actuated pump for cuff inflation, and algorithms and sensors to listen for initial and unrestricted arterial flow. However the cuff methods restrict blood flow during each measurement so they are unsuited to continuous use, and the determinations of blood pressure made by many automatic cuff systems fail to meet accuracy standards. The cuff also produces discomfort to the patient, which can influence blood pressure readings.
A-lines, which are used when continuous measurement is necessary, are reasonably accurate during periods free from signal artifact, from sources such as line-crimping, blood-clotting, and contact between the indwelling transducer and the arterial wall. However the transducer needs to be inserted surgically, and can cause thrombosis and infection. Because the method necessitates a surgical procedure, it is used sparingly, and frequently not recommended for use even when continuous pressure measurement would otherwise be desirable.
The experimental methods noted all attempt to circumvent the drawbacks of the A-line by measuring continuous blood pressure externally. Both direct external pressure sensing and indirect calculation methods have been devised.
The direct non-invasive methods use external pressure transduction. A pressure transducer is placed against an artery that lies just beneath the skin, such as the radial artery, and by pushing against the arterial wall senses pressure mechanically. However, because the transducer is sensing force, it is extremely subject to mechanical noise and motion artifact. Continuous measurement is problematical in that the transducer impedes blood flow. Difficulty also arises in keeping the transducer positioned properly over the artery. Thus, indirect-measurement methods have been considered.
Pulse-wave transit-time measurement is an indirect way of inferring arterial blood pressure from the velocity of the pulse wave produced at each heart cycle. however, though the velocity is related to blood pressure, the methods devised to date assume that the relationship is linear, and even if that were the case, it is probable that transit time by itself provides too little information about the pulse wave to permit the determination of blood pressure accurately. Another shortcoming of the method is that it is incapable of giving pressures at both systole and diastole, which many medical practitioners find useful.
Photoplethysmography, a technique of tracking arterial blood-volume and blood oxygen content, gives rise to the other indirect way of inferring blood pressure continuously. However, the methods based on it derive information from the volumetric data as though it were the same as blood pressure; that is, they assume that blood-pressure and blood-volume curves are similar--which is true sometimes but not in general. Furthermore, photoplethysmographic measurements are made at bodily extremities such as the earlobe or finger, and blood pressure observed at the body's periphery is not generally the same as from more central measurements.
Because the insertion of an A-line is frequently judged to be too invasive a procedure to undertake in order to determine blood pressure, and no practical non-surgical method of continuous measurement has yet supplanted it, the need for one remains.